1. Field of the Invention
The present invention relates to a method for preparing [F-18]-fluoride ion used for producing radiofluorinated organic compounds.
2. Related Art Statement
Radiofluorinated organic compounds are used in position emission tomography (PET), one of the medical diagnostic imaging technologies.
Currently, most radiofluorinated organic compounds are produced by organic chemical reactions of their precursors with [F-18]-fluoride ion. [F-18]-fluoride ion is formed by proton irradiation of [O-18]enriched water as target and obtained in the form of [O-18]-enriched water containing [F-18]-fluoride ion. To use the [F-18]-fluoride ion thus formed for labelling organic compounds, it is necessary to concentrate and separate [F-18]-fluoride ion from the [O-18]-enriched water containing [F-18]-fluoride ion obtained above. Furthermore, since [O-18]-enriched water is very expensive, it is desirable to recover and reuse [O-18]-enriched water after [F-18]-fluoride ion has been concentrated and separated therefrom.
A method of concentrating and separating [F18]-fluoride ion with the object of recovering and reusing [O-18]-enriched water has been already reported in the literature.
Schlyer et al. (Appl. Radiat. Isot., Vol. 41, No. 6, pp. 531-533 (1990)) reported that [O-18]-enriched water containing [F-18]-fluoride ion (about 0.5 ml) is brought into contact with a strongly basic anion exchange resin (hydroxyl form or carbonate form) to make [F-18]-fluoride ion adsorbed to the resin and, along therewith, to recover the [O-18]-enriched water which has passed through the resin and then the [F-18]-fluoride ion adsorbed to the strongly basic anion exchange resin is eluted with an aqueous solution of cesium carbonate or potassium carbonate, so that [F-18]-fluoride ion can be concentrated and separated at a collection efficiency of 95% or more. Hamacher et al. (Appl. Radiat. Isot., Vol. 41, No. 1, pp. 49-55 (1990)) reported that from about 2 ml of [O-18]-enriched water containing [F-18]-fluoride ion, in the same manner as above, [F-18]-fluoride ion can be concentrated and separated at a collection efficiency of 95% or more.
In the above-mentioned method, however, the recovered [O-18]-enriched water contains cations etc. as impurities, so that purification by distillation or the like is necessary to reuse the recovered [O-18]-enriched water for preparing radioactive fluoride ion.
On the other hand, with the increase of demand for radiofluorinated organic compounds, an increase in the production of [F-18]-fluoride ion has become necessary. Consequently, an increase in the amount of [O-18]-enriched water used in each production of [F-18]-fluoride ion is eagerly desired.
When it is intended, according to the method described above, to concentrate and separate [F-18]-fluoride ion from the larger amount of [O-18]-enriched water containing [F-18]-fluoride ion than that (about 2 ml) already reported in the literature, it may be assumed that the intention can be attained simply by increasing the amount of the resin. However, an increase in the amount of the resin makes it necessary to use an excess of aqueous basic solution as the eluent in eluting the adsorbed [F-18]-fluoride ion; this not only hinders the labelling reaction of organic compounds with the eluted [F-18]-fluoride ion but also is liable to lead to an increase in an amount of contamination with non-radioactive impurities.